The ability of the heart to perform work is essential for moving blood throughout the circulatory system and appears to be depressed in conditions of heart disease. The overall goal of this project is to investigate potential molecular mechanisms that regulate the work capacity of cardiac mycocytes. Experiments will specifically test how variable expression of the two cardiac myosin heavy chain (MyHC) isoforms regulates myocyte contractible properties. MyHC content will be manipulated over the entire range of 100 percent alpha-MyHC to 100 percent beta- MyHC by thyroid hormone-dependent expression of MyHC isoforms in rats. Definitive relationships between MyHC content and mechanical properties will be achieved by SDS-PAGE analysis of MyHC composition in single mycocytes following functional measurements. The first aim is to determine how altered alpha: beta MyHC ratios affect the important relationship between isometric force and [Ca2+]. The second aim will investigate how varied expression of alpha:beta MyHC ratios affect dynamic mechanical properties by correlating MyHC content with unloaded shortening velocities of the same individual myocyte. The third aim will determine the role that MyHC plays in determining power output by measuring power-load curves in myocytes expressing variable ratios of alpha:beta MyHC. Effects of variable expression of MyHC on power-load curves will also address the fourth aim, which is to provide insights into the steps(s) (i.e., detachment or attachment) of the myosin cross-bridge cycle that limit peak power output of myocytes. Further experiments will utilize reagents that increase specific cross-bridge states as a means assess which chemomechanical steps limit myocyte power output. The final aim to determine if the myocardial variations in both force and power output following protein kinase A (PKA)- induced phosphorylation differ between alpha-MyHC and beta-MyHC myocytes. Overall, these experiments should provide important insights into the role that cardiac MyHC plays in regulating myocardial contraction and determining the work capacity of individual myocytes. These experiments are important in elucidating the molecular mechanisms that determine work capacity of myocardium as well as improving our understanding of the functional consequences associated with reduced expression of alpha-MyHC as increased expression of beta-MyHC, which occurs during the progression of human heart failure.